1. Field of the Invention
The present invention relates to a ferrule for a plastic fiber and, more particularly, to a ferrule for use in hot plate treatment.
2. Description of the Prior Art
The thermal shock test (at a temperature from -40.degree. to 85.degree. C., for about 30 minutes) of a plastic fiber fitted in a ferrule has caused an end surface of the fiber to sink inwardly from an end of the ferrule because of shrinkage of the plastic fiber itself. The exact cause of the shrinkage of the fiber has not yet been clear. However, the shrinkage is considered to occur by the application of heat to a core/cladding (core wire) distorted due to heating and stretching when fabricated. Another cause of the shrinkage is different thermal expansion coefficients of the ferrule, a sheath material, and the core/cladding.
To prevent the fiber end surface from sinking, the plastic fiber has been fixed to the ferrule with an adhesive, and a front end of the plastic fiber has been melted down by means of a hot plate, which is known as hot plate treatment, so that the fiber end engages a front end portion of the ferrule for prevention of falling off.
For the hot plate treatment, a large-diameter portion 51 at a front end of a ferrule 50 and having an inner diameter greater than that of the ferrule 50 is required as a space for filling therein the plastic fiber melted and softened by means of the hot plate as shown in FIGS. 5A to 5C. The large-diameter portion 51 is basically of a rectangular cross-sectional configuration as shown in FIG. 5A. For relaxation of stresses to be applied during shrinkage, the large-diameter portion 51 has, in cross section, a rounded corner as shown in FIG. 5B and a chamfered corner as shown in FIG. 5C.
In the above stated hot plate treatment, a high-heat-resistant plastic fiber which is less susceptible to changes than general plastic fibers in hardness of the fiber at high temperatures is subjected to stress concentration on a portion of the fiber which is filled in the large-diameter portion of the ferrule due to fiber shrinkage. In particular, the thermal shock test has caused cracks in the stress concentrated portion.
In the plastic fiber fixed to the ferrule with an adhesive, the end surface of the fiber has not sunk inwardly from the end of the ferrule and no cracks have been generated, but it has been necessary to wait until the adhesive dries, resulting in poor productivity.